Power supply circuit

Abstract

A switching regulator includes a first full-wave rectifier for rectifying an input a.c. voltage, at least one power switch connected between the first rectifier and the primary winding of a pulse transformer whose secondary winding is connected through a second full-wave rectifier and a filter to a pair of output terminals across which a regulated d.c. output voltage appears, and a control circuit comprised of a level-shift circuit, a comparator and a pulse width modulator for controlling the on/off condition of the power switch, whereby the pulse width modulator supplies a pulse modulated signal to turn the power switch on only when the comparator supplies a high level signal to the pulse width modulator.

What is claimed is: 
     
       1. A power supply circuit for converting an a.c. input voltage into a d.c. output voltage comprising: first rectifying means which receives said a.c. input voltage and supplies across its first and second outputs a first rectified voltage rectified from said a.c. input voltage;   a transformer having a primary winding and a secondary winding;   switching means connected between the first output of said first rectifying means and one end of the primary winding the other end of which is connected to the second output of said first rectifying means;   second rectifying means connected to receive the voltage induced across the secondary winding of said transformer for supplying across its first and second outputs a second rectified voltage;   first filter means for filtering said second rectified voltage to generate said d.c. output voltage;   level-shift means connected to receive said first rectified voltage for producing a varying voltage which is proportional to said first rectified voltage, a first threshold voltage and a second threshold voltage which is lower in level than said first threshold voltage;   comparator means connected to receive said varying voltage and said first and second threshold voltages for supplying a comparator output signal which is either high or low depending upon whether said varying voltage is inbetween said first and second threshold voltages or not; and   pulse width modulating means having a first input for receiving a detecting signal indicating the level of said d.c. output voltage, a second input for receiving a reference signal and a third input for receiving said comparator output signal, said pulse width modulating means supplying a pulse width modulated output signal to said switching means only when said comparator output signal received is high.   
     
     
       2. A power supply circuit as in claim 1 wherein at least one of said first and second rectifying means includes a full wave rectifier. 
     
     
       3. A power supply circuit as in claim 2 wherein said full wave rectifier is a diode bridge rectifier. 
     
     
       4. A power supply circuit as in claim 1 wherein said level-shift means includes a voltage divider comprised of first and second resistors connected in series between the first and second outputs of said first rectifying means; a first buffer amplifier having its input connected to the junction between said first and second resistors and its output connected to supply said varying voltage to said comparator means; and a second buffer amplifier having its input connected to the junction between said first and second resistors and its output connected to threshold producing means. 
     
     
       5. A power supply circuit as in claim 1 wherein said first filter means includes a first inductor and a first capacitor which are connected in series between said first and second outputs of said second rectifying means whereby said d.c. output voltage is supplied across said first capacitor. 
     
     
       6. A power supply circuit as in claim 1 wherein said comparator output signal is high only when said varying signal is inbetween said first and second threshold voltages. 
     
     
       7. A power supply circuit as in claim 1 further comprising detecting means for detecting said d.c. output voltage to produce said detecting signal and feed-back means for supplying said detecting signal to said pulse modulating means. 
     
     
       8. A power supply circuit as in claim 7 wherein said feed-back means includes a photo-coupler thereby electrically isolating the primary side of said transformer from its secondary side. 
     
     
       9. A power supply circuit as in claim 1 wherein said reference signal is a high-frequency saw toothed waveform signal. 
     
     
       10. A power supply circuit for converting an a.c. input voltage into a d.c. output voltage comprising: first rectifying means which receives said a.c. input voltage and supplies across its first and second outputs a first rectified voltage rectified from said a.c. input voltage;   a transformer having a primary winding provided with a center tap connected to said first output of said first rectifying means and a secondary winding;   first switching means connected between said first output of said first rectifying means and one end of said primary winding;   second switching means connected between said second output of said first rectifying means and the other end of said primary winding;   second rectifying means connected to receive the voltage induced across the secondary winding of said transformer for supplying across its first and second outputs a second rectified voltage;   first filter means for filtering said second rectified voltage to generate said d.c. output voltage;   level-shift means connected to receive said first rectified voltage for producing a varying voltage which is proportional to said first rectified voltage, a first threshold voltage and a second threshold voltage which is lower in level than said first threshold voltage;   comparator means connected to receive said varying voltage and said first and second threshold voltages for supplying a comparator output signal which is either high or low depending upon whether said varying voltage is inbetween said first and second threshold voltages or not; and   pulse width modulating means having a first input for receiving a detecting signal indicating the level of said d.c. output voltage, a second input for receiving a reference signal and a third input for receiving said comparator output signal, said pulse width modulating means supplying first and second pulse width modulated output signals, which are out of phase from each other by 180 degrees, alternately to said first and second switching means only when said comparator output signal received is high.   
     
     
       11. A power supply circuit as in claim 10 wherein at least one of said first and second rectifying means includes a full wave rectifier. 
     
     
       12. A power supply circuit as in claim 11 wherein said full wave rectifier is a diode bridge rectifier. 
     
     
       13. A power supply circuit as in claim 10 wherein said level-shift means includes a voltage divider comprised of first and second resistors connected in series between the first and second outputs of said first rectifying means; a first buffer amplifier having its input connected to the junction between said first and second resistors and its output connected to supply said varying voltage to said comparator means; and a second buffer amplifier having its input connected to the junction between said first and second resistors and its output connected to threshold producing means. 
     
     
       14. A power supply circuit as in claim 10 wherein said first filter means includes a first inductor and a first capacitor which are connected in series between said first and second outputs of said second rectifying means whereby said d.c. output voltage is supplied across said first capacitor. 
     
     
       15. A power supply circuit as in claim 10 wherein said comparator output signal is high only when said varying signal is inbetween said first and second threshold voltages. 
     
     
       16. A power supply circuit as in claim 10 further comprising detecting means for detecting said d.c. output voltage to produce said detecting signal and feed-back means for supplying said detecting signal to said pulse modulating means. 
     
     
       17. A power supply circuit as in claim 16 wherein said feed-back means includes a photo-coupler thereby electrically isolating the primary side of said transformer from its secondary side. 
     
     
       18. A power supply circuit as in claim 10 wherein said reference signal is a high-frequency saw toothed waveform signal.